My spouse and I have been married for a year, together for five and they came out to me in January. They're identifying as genderqueer (at least for the moment) but present masculine and are wanting to transition to female but unsure how to begin.

From the girlfriend/wife perspective I can say this - your girlfriend needs to know. Tell her.

This isn't something you can wish away or forget as you've already experienced. When you're in a LTR with someone, they deserve to be involved in such life-altering decisions. I can say that when my spouse came out to me, it hurt. I was scared, confused, you name it, I felt it. But never once did I stop loving them. I don't know if I'm going to stay in my marriage through the transition... we're still figuring that out. I am straight. And while someone's gender/sex isn't the most important thing in a relationship, changing that fundamentally changes the nature of a relationship. Some people discover they're bi or flexible. Some people open up the relationship and seek intimacy elsewhere. Some people just don't care about sex. I personally don't fit any of those categories, unfortunately for me. My spouse deserves someone who will love them completely in their new identity (after they transition).

BUT! Even though the romantic relationship I have with my spouse may come to an end, I KNOW, yes I KNOW we will still care for each other and be friends. Throughout this whole thing I have still felt better about us than I have in other relationships that ended for completely typical reasons (growing apart, cheating, etc). I can remember the worst breakup I ever had with a boyfriend... how much I felt tossed aside and hurt and how I hated him. Jerk. I feel NOTHING like that towards my current spouse. I prefer this "ending" of a relationship to the one I experienced in the past.

Good luck to you! Take the time to put your thoughts together and then tell her. Don't assume the worst will happen. As long as she understands this is not about her and not her fault, she will be okay.